1. Field of the Invention
The present invention relates to a suspension mechanism for hanging an object over another object and, more particularly, to such a suspension mechanism which is particularly suitable as means for supporting X-ray tube in an X-ray diagnostic device which is to be hung over a patient.
2. Description of the Background Art
Conventionally, a suspension mechanism utilized in an X-ray diagnostic device for supporting an X-ray tube in vertically movable manner is equipped with springs for providing a force that balances gravitational force acting on a suspended object. Such springs can be stretchable solenoid type, compressible solenoid type, or windup type, but for the reason of compactness the windup type is preferred most.
FIG. 1 shows an example of such a suspension mechanism in an X-ray diagnostic device, in which a patient 1 is laid on a bed 2, over an X-ray film 3, and a suspended object 4 including an X-ray tube is suspended over the patient 1 by a suspension wire 6 in vertically movable manner which is connected to a balance mechanism 7 through an idler pulley 8. The suspension wire 6, balance mechanism 7 and idler pulley 8 are housed inside a frame 5 attached on a ceiling 9.
FIG. 2 shows a detail of the balance mechanism 7, in which the suspended object 4 is suspended by left and right suspension wires 6A and 6B to be wound up by a corn pulley 11 attached on a shaft 12 which can rotate by means of windup type springs 13A and 13B (not shown in FIG. 2) contained inside left and right barrels 10A and 10B fixed to the frame 5 by fixing members 27A and 27B, respectively.
The windup type springs 13A and 13B inside the barrels 10A and 10B are shown in FIGS. 3(A) and 3(B). As shown in FIG. 3(B), one ends of the windup type springs 13A and 13B are hooked on the barrels 10A and 10B, respectively, while the other ends are hooked on the shaft 12, so that torque due to the windup type springs 13A and 13B will be transformed into a power to wind the suspension wires 6A and 6B around the corn pulley 11. Here, the torque due to the windup type springs 13A and 13B, by nature of this type of spring, changes as the shaft 12 rotates, but the corn pulley 11 with changing radii offsets the change in torque, so that the suspended object 4 can be lifted at a constant rate.
Two suspension wires 6A and 6B are provided for the purpose of safety, so that even when one of them are cut off the other one can keep the suspended object 4 from falling. This feature is particularly important in this type of the X-ray diagnostic device in which the X-ray tube have to be suspended over the patient 1, since the fall of the suspended object 4 can cause a serious trouble in such a configuration.
Such a conventional suspension mechanism has two serious problems.
One is that featuring two suspension wires 6A and 6B necessitates double threads on the corn pulley 11, but such double threads increases a fleet angle, which in turn shorten the durability of the suspension wires 6A and 6B.
Namely, because of the configuration of the corn pulley 11, location of the suspension wires 6A and 6B slides along the shaft 12 in a direction indicated by an arrow A in FIG. 2, so that when the suspension wires 6A and 6B make contact with the idler pulley 8, the direction of the suspension wires 6A and 6B changes from the beginning of winding to the end of the winding by a so called fleet angle .THETA., as can be seen from FIGS. 4(A) and 4(B).
A situation for the double threaded corn pulley 11 is shown in detail in FIG. 5. Here, L.sub.1 and L.sub.2 indicate directions of the one of the suspension wires 6A and 6B wound along the thread B at the beginning and end of winding, respectively, while .THETA..sub.1 indicates the fleet angle for this suspension wire. In FIG. 5, the other suspension wire wound along another thread C is not depicted for the sake of avoiding confusion. When the pitch between a pair of threads B and C is P.sub.1, the pitch between adjacent double threads is P.sub.2, the distance between the corn pulley 11 and the idler pulley 8 is S, and the winding number of the corn pulley 11 is N, the fleet angle .THETA..sub.1 can be expressed as: EQU .THETA..sub.1 =tan.sup.-1 {(P.sub.1 +P.sub.2).times.N/2.times.1/S}(1)
so that the fleet angle .THETA..sub.1 increases as products of P.sub.1 and P.sub.2 with N increases, which is the case of the double threaded one compared with a single threaded one. Since the larger fleet angle .THETA..sub.1 creates more frictional contact between the suspension wires 6A and 6B and the idler pulley 8, the durability of the suspension wires 6A and 6B becomes more restricted for the double threaded corn pulley 11.
Another problem of a conventional suspension mechanism is that the windup type springs 13A and 13B can be broken by fatigue due to repeated use. Of course the windup type springs 13A and 13B are designed to be durable enough for practical purposes, but the breaking of the windup type springs 13A and 13B in an unexpectedly short period of time is known to be caused by the use of defective material and the surface damages.
When one of the windup type spring 13A and 13B is broken, since the other one cannot maintain the balance with the suspended object 4 all by itself, the falling of the suspended object 4 results.
As already mentioned, the elimination of the possibility for such a fall of the suspended object 4 is a crucial safety requirement for an X-ray diagnostic device of type shown in FIG. 1. But, in a conventional suspension mechanism, the use of otherwise advantageous windup type springs made it difficult to eliminate possibility for the suspended object's fall due to the breaking of windup type springs.